Mechanism of metallic conduction: Drift velocity

In any metallic conductor large no of free electrons are available these electrons are in random motion as a result of which their average velocity is zero and there is no net flow of electrons so current is zero.

When the conductor is connected to an external battery, an electric field is setup in the conductor and the electron experience electrostatic force and accelerate opposite direction of electric field. These electrons collide with neighboring atoms and ions. Due to continuous collision their average acceleration becomes zero  and electrons acquire constant average velocity called drift velocity. 

Hence the constant average velocity acquired by the electrons in any conductor subjected to an electric field is called drift velocity. 

Relation between Drift Velocity and Current